Method of forming a device by removing a conductive layer of a wafer

ABSTRACT

A method of forming a MEMS device provides a wafer having a base with a conductive portion. The wafer also has an intermediate conductive layer. After it provides the wafer, the method adds a diaphragm layer to the wafer. The method removes at least a portion of the intermediate conductive layer to form a cavity between the diaphragm layer and the base. At least a portion of the diaphragm layer is movable relative to the base. After it forms the cavity, the method seals the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/049,205filed Feb. 2, 2005, now U.S. Pat. No. 7,491,566, which claims priorityfrom U.S. Provisional Application No. 60/543,059, filed Feb. 9, 2004,entitled, “PRESSURE SENSOR,” the disclosures of which are incorporatedby reference herein in their entireties.

FIELD OF THE INVENTION

The invention generally relates to MEMS devices and, more particularly,the invention relates to MEMS pressure sensors.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (“MEMS,” hereinafter “MEMS devices”) areused in a wide variety of applications. For example, MEMS devicescurrently are implemented as microphones to convert audible signals toelectrical signals, as gyroscopes to detect pitch angles of airplanes,and as pressure sensors to detect changes in pressure. In simplifiedterms, such MEMS devices typically have a movable structure suspendedfrom a substrate, and associated circuitry (either on or off chip) thatboth senses movement of the suspended structure and delivers the sensedmovement data to one or more external devices (e.g., an externalcomputer). The external device processes the sensed data to calculatethe property being measured (e.g., pressure).

To form pressure sensors, many prior art processes form a cavity betweena movable diaphragm and a base. To form the cavity, various processesknown to the inventors remove an insulator material, which requires HFetches of a sacrificial oxide. Such etches, however, can createfabrication inefficiencies. Specifically, HF etches can harmmetallization and pre-formed circuits, while causing stiction incompliant structures. In addition, such processes are relatively slowand thus, often have difficulty clearing out large cavities.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method of forming aMEMS device provides a wafer having a base, a first conductive layer, asecond conductive layer, and an intermediate conductive layer. After itprovides the wafer, the method removes at least a portion of theintermediate conductive layer to form a cavity between the first andsecond conductive layers. At least a portion of the first conductivelayer is movable relative to the base to form a diaphragm, while thesecond conductive layer is substantially immovable relative to the base.After it forms the cavity, the method seals the cavity.

In illustrative embodiments, the first and second conductive layers forma variable capacitor that fluctuates as the first conductive layermoves. Moreover, the cavity may be sealed to have a cavity pressure thatis less than atmospheric pressure. Among other materials, the first andsecond conductive layers may be a silicon based material (e.g., silicon,polysilicon, or silicon germanium).

The method may add circuitry to the wafer before removing theintermediate conductive layer. Among other things, the circuitry may becapable of detecting movement of the diaphragm relative to the secondconductive layer. The method may remove the intermediate layer in anumber of ways, such as by applying a dry gas phase etch to theintermediate conductive layer. In addition, the first conductive layer,intermediate conductive layer, and second conductive layer may be formedfrom substantially the same material. In that case, the wafer may have abarrier layer of different material between the first conductive layerand the intermediate conductive layer to facilitate removal of theintermediate layer.

In accordance with another aspect of the invention, a method of forminga MEMS device provides a wafer having an intermediate conductive layerand a base with a conductive portion. The method then adds a diaphragmlayer to the wafer, and removes at least a portion of the intermediateconductive layer to form a cavity between the diaphragm layer and thebase. At least a portion of the diaphragm conductive layer is movablerelative to the base. The method then seals the cavity.

Among other things, the base includes a support substrate that supportsa conductive layer that is part of the conductive portion. In addition,the method may dice the wafer to form a plurality of like devices.Circuitry may be added to the wafer before removing at least a portionof the intermediate conductive layer. Alternatively, circuitry could beadded at a later time.

In some embodiments, the wafer has additional material supported by theintermediate conductive layer. In that case, the intermediate conductivelayer may be between the additional material and the conductive portionof the base. The method thus may remove the additional material beforeadding the diaphragm conductive layer.

In accordance with another aspect of the invention, a method of forminga pressure sensor provides a wafer having an intermediate conductivelayer and a base with a conductive portion. The method removes at leasta portion of the intermediate conductive layer to form a cavity, andthen seals the cavity.

The method may add a diaphragm layer to the wafer, at least a portion ofwhich is movable relative to the base after the cavity is formed.Alternatively, the wafer has a diaphragm layer when provided. Moreover,the wafer may have circuitry when provided, or be free of circuitry whenprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and advantages of the invention will be appreciated morefully from the following further description thereof with reference tothe accompanying drawings wherein:

FIG. 1 schematically shows a packaged integrated circuit or chip thatmay be produced in accordance with illustrative embodiment of theinvention.

FIG. 2 shows a process of forming a pressure sensor in accordance withillustrative embodiments of the invention.

FIG. 3 schematically shows a top view of a wafer that may be used in theprocess shown in FIG. 2.

FIGS. 4A-4C schematically show details of the process shown in FIG. 2 inaccordance with an embodiment that uses an internal layer as adiaphragm.

FIGS. 5A-5D schematically show details of the process shown in FIG. 2 inaccordance with an embodiment that requires a deposited diaphragm.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the invention fabricate a MEMS device (e.g.,a pressure sensor) from existing layers of a multilayered wafer. Suchembodiments typically use one or more conductive layers as a sacrificiallayer. Details of illustrative embodiments are discussed below.

FIG. 1 schematically shows an exemplary use of a chip 10 produced inaccordance with illustrative embodiments of the invention. Specifically,the chip 10 in this embodiment is a MEMS device having both circuitry 11and movable structure 13 (see FIGS. 3, 4A-4C, and 5A-5D). The MEMSdevice 10 illustratively is formed on a silicon wafer and packagedwithin a conventional package 12. The package 12 is coupled with acircuit board 14 having interconnects 16 to electrically communicatewith an external device, such as a computer.

Since it is implemented as a MEMS device, the chip 10 may execute anyconventionally known functionality commonly implemented by a MEMSdevice, such as a pressure sensor. Although the chip 10 is discussed asa MEMS pressure sensor, principles of illustrative embodiments can applyto other chips or integrated circuits, such as MEMS inertial sensors andmicrophones. Accordingly, discussion of a pressure sensor is exemplaryand not intended to limit the scope of various embodiments of theinvention.

It should be noted, however, that discussion of MEMS devices isexemplary. Accordingly, principles of illustrative embodiments may applyto other types of chips or integrated circuits.

FIG. 2 schematically shows a process of fabricating the chip 10 shown inFIG. 1. The process begins at step 200, which provides a foundry waferto be processed into a plurality of pressure sensors. FIG. 3schematically shows an exemplary foundry wafer 18, which has an array ofpreviously fabricated sets of circuits. Of course, the circuitsillustratively are preconfigured to cooperate with the MEMS structure 13to be formed on the wafer 18. For example, the circuitry 11 may beconventional CMOS circuitry implementing pressure sensor functionality.Each set of circuits illustratively is for a single chip 10 that will beformed after a subsequent singulation step. As known by those skilled inthe art, such a wafer typically has a plurality of conductive layersseparated by one or more insulator layers 24.

It should be noted that identifying the wafer 18 as a “foundry” wafer isnot intended to limit the scope of various embodiments. In fact, someembodiments of the invention use generic multi-layered wafers having aplurality of conductors formed from the same or different materials. Inaddition, among other things, any of the conductive layers may be formedfrom some non-insulating material, such as a semiconductor (doped orundoped polysilicon) or metal. Moreover, although the wafer 18 shown inFIG. 3 has prefabricated circuitry 11, some embodiments do not form anycircuitry on the wafer 18 before step 200.

FIGS. 4A-4C and 5A-5D illustrate the wafer 18 at various stages of theprocess shown in FIG. 2. Each set of figures, however, shows a differentembodiment. In particular, FIGS. 4A-4C show simplified cross-sectionalviews of one embodiment that does not require a deposited conductivematerial to act as a diaphragm, while FIGS. 5A-5D show simplifiedcross-sectional views of another embodiment that requires a depositedlayer to act as a diaphragm. To simplify this discussion, however, thecircuitry 11 is not shown in any of FIG. 4A-4C or 5A-5D. In fact, itshould be noted that these figures are exemplary and not intended toencompass all embodiments of the invention. Accordingly, the wafer 18may have additional layers or components not shown in the figures.

More specifically, FIG. 4A schematically shows an embodiment in whichthe wafer 18 has a base layer 20 with a conductive region 22, and aninsulator 24 containing two additional conductive layers 26A and 26B.The insulator 24 may be considered to form various insulator layers 24between the conductive layers 26A and 26B and base 20. In addition, thewafer 18 also has conductive interconnects 30A and 30B (e.g., formedfrom metal) that respectively permit electrical communication for theconductive layer 26B and conductive region 22. In illustrativeembodiments, the conductive layers 26A and 26B are formed from metal ora silicon based material (e.g., doped or undoped polysilicon or silicongermanium). Moreover, the conductive region 22 of the base layer 20 maybe doped silicon. In place of or in addition to the conductive region 22of the base layer 20, the wafer 18 may have an additional conductivelayer (not shown) abutting or near the top surface of the base layer 20.

FIG. 5A schematically shows a similar embodiment of the provided wafer18, which has a base layer 20 and an insulator 24 also containing twoadditional conductive layers 26C and 26D. In a manner similar to theabove discussed embodiment, the insulator 24 may be considered to formvarious insulator layers 24 between the conductive layers 26C and 26Dand the base 20. The wafer 18 also has a first conductive interconnect30C (e.g., formed from metal) that permits electrical communication forone of conductive layers 26D, and a second metal interconnect 30D thatpermits electrical communication for a subsequently deposited layer(discussed below). Also similar to the above discussed embodiment, theconductive layers 26C and 26D within the insulator may be formed frommetal or a silicon based material.

The process continues to step 202, which removes some of the top layersof the wafer 18. For example, illustrative embodiments remove circuitdielectrics (e.g., insulator layers 24) down to a specified level. Forexample, in FIGS. 4B and 5B, the insulator material respectively betweenthe top wafer surface and the top conductive layers 26B and 26C isremoved. The embodiment in FIG. 4B, however, exposes the top surface ofthe top conductive layer 26B, while the embodiment in FIG. 5B leaves athin layer of insulator material 24 on the top conductor 26C.

Before, during, or after execution of step 202, the process forms etchholes 32 that each extend to one (or more, as the case may be) of theconductive layers in the wafer 18 (step 204). For the embodiment shownin FIG. 4B, the etch holes 32 extend to the lower conductive layer 26A.For the embodiment shown in FIG. 5B, however, the etch holes 32 extendto the top conductive layer 26C.

It then is determined at step 206 if an existing layer is to be used asthe diaphragm. As noted above, this is an important distinction betweenthe embodiments shown in FIGS. 4A-4C and 5A-5D. If existing layers arenot to be used in this manner, then the process deposits and patterns adeposited layer 34 in the previously removed regions of the wafer 18(step 208). For example, as shown in FIG. 5C, the deposited layer 34 isdeposited on the insulator layer 24 remaining after the top layers wereremoved. As shown in FIG. 5C, the deposited layer 34 is deposited tomake electrical contact with the second metal interconnect 30D, thuspermitting electrical communication with circuit elements. Inillustrative embodiments, the deposited layer 34 is a flexible andconductive material. Among other things, it may be a metal orsilicon-based material, such as silicon-germanium or polysilicon (dopedor undoped).

Returning to decision step 206, if an existing layer is to be used as adiaphragm, the process skips step 208 and also continues to step 210.Specifically, at step 210, the process removes one or more of theconductive layers from the wafer 18. Because they are removed, such aconductive layer (referred to as the “sacrificial layer”) consequentlyshould not serve as an interconnect for electronic components or MEMSstructure 13.

For the embodiment shown in FIG. 4C, the process removes the lowerconductive layer 26A only. This is in contrast to the embodiment shownin FIG. 5D, in which the process removes the upper conductive layer 26Conly. In either case, removal of the sacrificial layer produces a cavity35 into which the diaphragm may flex. Specifically, the top conductivelayer 26B forms the diaphragm in the embodiment shown in FIG. 4C, whilethe deposited layer 34 forms a diaphragm in the embodiment shown in FIG.5D. The diaphragms of the two embodiments each form a variable capacitorwith the lower conductive layer 26D and conductive region 22,respectively. Accordingly, the lower conductive layer 26D and conductiveregion 22 respectively form substantially stationary plates of theirvariable capacitors, while the diaphragms (i.e., conductive layer 26Band deposited layer 34) respectively form movable plates for theircapacitors. During use, the amount of diaphragm flexure, and thuschanging capacitance, determines the pressure reading.

Illustrative embodiments remove the sacrificial layer by means of a drygas phase etch. For example, xenon difluoride may be applied to thesacrificial layers that are formed from polysilicon. The insulatormaterial around the sacrificial layer prevents the xenon difluoride fromaffecting other conductive layers. The insulator material therefore actsas a barrier for the other conductive layers. Such a barrier should notbe necessary if any of the other conductive layers is not sensitive tothe xenon difluoride (e.g., if the sacrificial layer is formed from adifferent material than that forming the other conductive layers). Ifthe sacrificial layers are formed from metal, however, illustrativeembodiments may use a wet metal etch.

After removing the sacrificial layers, the process seals the cavity 35by depositing plugs 36 over the etch holes 32 (step 212). This sealillustratively is hermetic. To those ends, the process may deposit anitride layer over portions of the top surface of the wafer 18. At aminimum, this nitride layer should at least partially fill (and thusseal) the etch holes 32. In some embodiments, this nitride layer alsocovers the diaphragm, electrical contacts, and other top facingcomponents. It nevertheless may be advantageous or necessary to removethe nitride from various of those components, such as the electricalcontacts. In addition, this nitride layer also may serve as an in-situcap for other portions of the wafer 18.

In illustrative embodiments, the process seals the etch holes 32 in anenvironment having a pressure that is lower than atmospheric pressure.For example, the etch holes 32 may be sealed in a vacuum. Consequently,during anticipated use, the pressure within the cavity 35 should belower than external pressures and thus, be relatively sensitive topressure changes. Those skilled in the art can select an appropriatecavity pressure based upon the desired sensitivity of the pressuresensor.

At this point in the process, circuitry is added to those embodimentsthat do not yet have circuitry. As noted above, the circuitry may beconventionally known circuitry for the given application and formed in aconventional manner. In fact, such embodiments may add circuitry at anypoint in the process before the chip 10 is packaged. Alternatively, someembodiments may have no circuitry. Accordingly, such embodiments relyupon off-chip circuitry to perform their function.

After the etch holes 32 are plugged, the process continues to step 214,which singluates the wafer 18. Conventional sawing/dicing processes maybe used. Finally, the process ends by packaging the chip 10 in aconventional package 12 (step 216). A number of different package typesmay be used, such as ceramic packages or premolded packages.

Accordingly, this process forms a plurality of individual MEMS pressuresensors from a single multilayer wafer. The physical location of thecircuitry 11 and MEMS structure 13 can be selected based upon therequirements of the application. For example, the circuitry 11 maycircumscribe the MEMS structure 13. As a second example, the MEMSstructure 13 may be on one portion of the chip 10, while the circuitry11 may be on another portion of the chip 10.

It should be noted that the process discussed with regard to FIG. 2 isnot intended to be complete with regard to all possible steps forproducing a chip 10. Instead, the process highlights various importantsteps for implementing illustrative embodiments of the invention. Inaddition, some of the steps of the process can be executed in adifferent order, or at substantially the same time. For example, theembodiment shown in FIGS. 4A-4C may form the etch holes 32 beforeremoving the top layers. In fact, that embodiment also may remove thesacrificial layer before removing the top layers. As a further example,the diaphragm may be deposited (step 208) before forming the etch holes22 (step 204).

Accordingly, illustrative embodiments of the invention can moreefficiently form an integrated circuit, such as a MEMS pressure sensor,from existing layers of a multilayered wafer. In fact, the wafer canalready have prefabricated circuitry that cooperates with subsequentlyformed structure. Moreover, to improve processing efficiency andultimate product robustness, illustrative embodiments form internalcavities (or release MEMS structure) by using conductive layers assacrificial material.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of forming a MEMS device, the method comprising: providing awafer having a base and an insulator having an upper conductive layer, alower conductive layer, and a dielectric layer on top of the upperconductive layer; removing at least a portion of the dielectric layer inregions above the upper conductive layer; adding a diaphragm layer tothe wafer in at least a portion of the removed regions; removing atleast a portion of the upper conductive layer to form a cavity betweenthe diaphragm layer and the base, at least a portion of the diaphragmlayer being movable relative to the base; and sealing the cavity.
 2. Themethod as defined by claim 1 further including dicing the wafer to forma plurality of like devices.
 3. The method as defined by claim 1 furthercomprising adding circuitry to the wafer before removing at least aportion of the upper conductive layer.
 4. The method as defined by claim3 wherein the circuitry is capable of detecting movement of thediaphragm relative to the lower conductive layer.
 5. The method asdefined by claim 1 wherein the diaphragm layer and the lower conductivelayer form a variable capacitor that fluctuates as the portion of thediaphragm layer moves relative to the base.
 6. The method as defined byclaim 1 wherein sealing comprises sealing the cavity to have a cavitypressure that is less than atmospheric pressure.
 7. The method asdefined by claim 1 wherein the diaphragm layer and the lower conductivelayer comprise a silicon based material.
 8. The method as defined byclaim 1 wherein removing includes applying a dry gas phase etch to theupper conductive layer.
 9. The method as defined by claim 1 wherein thelower conductive layer, the upper conductive layer, and the diaphragmlayer are formed from substantially the same material, the wafer havinga barrier layer of different material between the lower conductive layerand the upper conductive layer.
 10. The method as defined by claim 1wherein the wafer has circuitry when provided.
 11. The method as definedby claim 1 wherein the wafer is free of circuitry when provided.
 12. Themethod as defined by claim 1 wherein the upper conductive layercomprises a semiconductor.
 13. The apparatus formed by the methoddefined by claim 1.